SOI wafer and method for producing the same

ABSTRACT

The present invention provides a SOI wafer produced by an ion implantation delamination method wherein a width of a SOI island region in a terrace portion generated in an edge portion of the SOI wafer where a surface of a base wafer is exposed is narrower than 1 mm and a density of pit-shaped defects having a size of 0.19 μm or more existing in a surface of a SOI layer detected by a LPD inspection is 1 counts/cm 2  or less, and also provides a method for producing the SOI wafer. Thereby, there is provided a SOI wafer produced by an ion implantation delamination method wherein generation of SOI islands generated in delamination can be suppressed and a defect density of LPDs existing in a surface of the SOI wafer can be reduced, and a method for producing the same, so that device failure can be reduced.

This application is a 371 of PCT/JP02/03162 filed Mar. 29, 2002 and thisapplication claims the benefit under 35 U.S.C. §I19(e) of prior toforeign application JAPAN 2001-108643 filed Apr. 6, 2001.

TECHNICAL FIELD

The present invention relates to a method for producing a SOI wafer bybonding of an ion-implanted wafer and a wafer and delamination,so-called ion implantation delamination method (also called a hydrogenion delamination method or smart-cut method), which can suppressgeneration of SOI islands generated in a terrace portion and generationof LPD generated in a surface of the SOI wafer, so that device failurecan be reduced.

BACKGROUND ART

The ion implantation delamination method is a method of producing a SOIwafer by bonding of a wafer into which hydrogen ions or rare gas ionsand a wafer and delamination. However, there is a case where a SOI layeris not transferred on an edge portion of the SOI wafer afterdelamination, and accordingly a terrace portion where a surface of abase wafer (support substrate) is exposed is generated. This is mainlydue to that the bonding strength between the bonded wafers is weak inthe edge portion of the wafers, and therefore, a SOI layer is difficultto be transferred on a base wafer side. When the SOI terrace portion wasobserved by a optical microscope, it was found that SOI islands whichare pieces of isolated island-shaped SOI layer were generated in theedge portion of the SOI layer. It is expected that such SOI islands comeoff from the wafer by etching to eliminate a buried oxide film (may becalled a BOX oxide film) during a cleaning by use of an aqueous solutioncontaining HF (hydrofluoric acid) in a device fabrication process, andaccordingly, they cause device failure because they adhere to a devicefabrication region of the wafer again as silicon particles.

FIG. 1 show cross sectional views of an edge portion of the SOI waferproduced by an ion implantation delamination method.

FIG. 1( a) is a SOI wafer 10, and FIG. 1( b) shows the detail of itsedge portion. FIG. 1( b) schematically shows the state that the SOIwafer 10 comprises a SOI layer 25, a buried oxide film 26 and a basewafer 27, and a terrace portion 43 where a surface of the base wafer isexposed and SOI islands 42 which are pieces of isolated island-shapedSOI layer are generated in the edge portion.

On the other hand, as SOI wafers produced by the ion implantationdelamination method have been observed by an optical surface inspectionapparatus, it has been found that defects detected as LPD exist. The LPD(Light Point Defect) is a generic term of the defect, which appears as aspot when a surface of the wafer is observed under a condenser lamp.Although this defect has not been identified, it is considered that adevice yield is affected because nearly all the defects are shallow pitsand become holes which pass through a SOI layer when a SOI layer isoxidized to be thin.

DISCLOSURE OF THE INVENTION

The present invention has been accomplished in view of theabove-mentioned problems, and a main object of the present invention isto provide a SOI wafer produced by an ion implantation delaminationmethod wherein generation of SOI islands generated in delamination aresuppressed and the defect density of LPD existing in a surface of theSOI wafer is reduced, and also provide a method for producing the same,so that the device failure is reduced.

In order to solve the above problems, the present invention provides aSOI wafer produced by an ion implantation delamination method wherein awidth of a SOI island region in a terrace portion generated in an edgeportion of the SOI wafer where a surface of a base wafer is exposed isnarrower than 1 mm.

As described above, in the present invention, the SOI wafer of whichwidth of a SOI island region in a terrace portion generated in an edgeportion where a surface of a base wafer is exposed is narrower than 1 mmcan be obtained. And if the width of a SOI island region is narrowerthan 1 mm like this, there will be few cases where SOI islands come offfrom the wafer by etching to eliminate a buried oxide film during a HFcleaning in a device fabrication process, and they cause device failureon the ground that they reattached as silicon particles to a devicefabrication region. Accordingly, the device yield can be improved.

Further, the SOI wafer according to the present invention is the SOIwafer produced by an ion implantation delamination method wherein adensity of pit-shaped defects having a size of 0.19 μm or more existingin a surface of a SOI layer detected by a LPD inspection is 1 counts/cm²or less.

As described above, in the present invention, a SOI wafer havingextremely few pit-shaped defects can be obtained. And if a density ofpit-shaped defects having a size of 0.19 μm or more existing in asurface of a SOI layer detected as a LPD is 1 counts/cm² or less, devicefailure can be reduced, and thus, device yield can be improved.

Next, a method for producing a SOI wafer according to the presentinvention is the method for producing a SOI wafer by an ion implantationdelamination method wherein a lower limit of an implantation dosage ofhydrogen ions or rare gas ions is determined by a delaminationphenomenon, and an upper limit of an implantation dosage of hydrogenions or rare gas ions is determined by a width of a SOI island region ina terrace portion or a density of pit-shaped defects detected by a LPDinspection of the SOI wafer.

If the implantation dosage of hydrogen ions or rare gas ions isdetermined by this manner and the ions are implanted, the SOI wafer suchthat generation of SOI islands, which is easily generated indelamination, are suppressed or a defect density of LPD existing in asurface of the SOI wafer is reduced can be produced.

Specifically, the implantation dosage of hydrogen ions is 5×10¹⁶ions/cm² or more and less than 7.5×10¹⁶ ions/cm².

According to this method, since a width of a region of SOI islands whichare easily generated in delamination can be reduced to 1 mm or less, theSOI wafer of which a SOI layer can be certainly transferred to a basewafer and a defect density of LPD existing in a surface is extremelyreduced can be produced. Further, by determining the implantation dosagein the above range, the wafer can be stably delaminated.

As described above, according to the present invention, a SOI waferproduced by an ion implantation delamination method wherein generationof SOI islands generated in delamination is suppressed and a defectdensity of LPD existing in a surface of the SOI wafer is reduced and amethod for producing the same can be provided, and accordingly devicefailure can be reduced.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 include schematic diagrams showing a terrace and SOI islandsgenerating in an edge portion of a SOI wafer;

-   -   (a) A SOI wafer, and (b) an edge portion of the SOI wafer.

FIG. 2 includes optical microscope photographs showing variations of aSOI island region width and a terrace width depending on the ionimplantation dosages (ions/cm²);

-   -   (a) 5.5×10¹⁶, (b) 6.5×10¹⁶, and (c) 7.5×10¹⁶.

FIG. 3 is a diagram showing variations of LPD densities (counts/cm²) inwafer surfaces relative to ion implantation dosages.

FIG. 4 is a flow chart showing one example of a production process of aSOI wafer according to the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, the present invention will now be described in detail.

Inventors of the present invention found that, in order to produce a SOIwafer by an ion implantation delamination method wherein generation ofSOI islands generated in delamination is suppressed or a defect densityof LPD existing in a surface of the SOI wafer is reduced, it issufficient to control an implantation dosage of hydrogen ions or raregas ions within a predetermined range, and they carefully examined itsconditions and accomplished the present invention.

A so-called SOI island, which remains at a certain width in a terraceportion generated in an edge portion of the SOI wafer where a surface ofa base wafer is exposed, can be observed after delamination heattreatment. And it is considered that delamination in an ion implantationdelamination method occurs due to a growth of defects induced byhydrogen or rare gas generated during delamination heat treatment and astrength of rapid volume expansion due to vaporization of implantedhydrogen or rare gas. And it is also considered that in the edge portionof the wafer having the terrace portion, generation of SOI islands ismainly due to that a bonding strength between the bonded wafers is weakunder the influence of polishing sags in each edge portion of wafersurfaces to be bonded, and a SOI layer is difficult to be transferred toa base wafer side.

In consideration of such characteristics of a delamination phenomenon,inventors of the present invention found that if an implantation dosageof hydrogen ions is reduced less than a conventional dosage of 8×10¹⁶ions/cm², a width of a SOI island region can be reduced.

Namely, if the implantation dosage of hydrogen ions is 5×10¹⁶ ions/cm²or more and less than 7.5×10¹⁶/cm², the width of a SOI island region canbe 1 mm or less and a density of pit-shaped defects having a size of0.19 μm or more existing in a surface of a SOI layer detected by a LPDinspection can be 1 counts/cm² or less.

The reason why the width of a SOI island region is extended when theimplantation dosage of hydrogen ions is 7.5×10¹⁶ ions/cm² or more is notclear up to the present. However, it is considered that, if animplantation dosage is larger than necessary, delamination occurs ashydrogen induced defects are insufficiently grown since the strength ofexpansion due to vaporization of hydrogen is too strong, and thus, theregion where delamination occurs in the state that there is noconnection in a lateral direction like SOI islands is generated.

Further, if a dosage of hydrogen is less than 5×10¹⁶ ions/cm², formationof defects and strength of expansion due to vaporization of hydrogen isnot sufficient since the implantation dosage of hydrogen is too small.Thus, a SOI layer is only partially transferred to a base wafer, thewhole surface of the wafer is not delaminated. Therefore, it was foundthat the SOI wafer can not be made up. Consequently, it is necessary tocontrol the dosage of hydrogen to 5×10¹⁶ ions/cm² or more and less than7.5×10¹⁶ ions/cm² in the ion implantation delamination method. And inorder to stably perform delamination, the implantation dosage ispreferably 5.5×10¹⁶ ions/cm² or more, more preferably more than 6×10¹⁶ions/cm² to produce a SOI wafer.

As described above, as to the implantation dosage of hydrogen ions, theexistence of the optimum range of the dosage was revealed. And, inconsideration of the above mechanism, it can be supposed that thisphenomenon is also applied to the case where not only hydrogen ions butalso rare gas ions are implanted.

Further, when a surface of the SOI layer was observed by an opticalsurface inspection apparatus, it was found that a density of defectsdetected as LPD depends on an implantation dosage of hydrogen ions, andwhen the implantation dosage is 7.5×10¹⁶ ions/cm² or more, LPDs areextremely increased. Although LPDs existed about 2 counts/cm² or morewhen the conventional implantation dosage of 8×10¹⁶ ions/cm² wasimplanted, LPDs can be sharply decreased to 1 counts/cm² or less.

As described above, in the present invention, an lower limit of theimplantation dosage of hydrogen ions or rare gas ions is determined by adelamination phenomenon, and an upper limit of the implantation dosageof hydrogen ions or rare gas ions is determined by a width of a SOIisland region in the terrace portion or a density of pit-shaped defectsdetected by a LPD inspection of the SOI wafer.

Hereinafter, the present invention will be further explained in detailwith reference to the drawings.

FIG. 4 is a flow chart showing one example of the production process ofa SOI wafer in the ion implantation delamination method in which ahydrogen ion implanted wafer is bonded to a wafer and delaminated.

Hereinafter, the case of bonding two silicon wafers will be mainlyexplained.

The ion implantation delamination method in which a SOI wafer isproduced by bonding of an ion-implanted wafer and a wafer anddelamination has, for example, two methods of A and B of which aredifferent in the order of processing steps. First, the method A will beexplained.

In the process 1 of the method A, two mirror-polished silicon wafers arerequired to be prepared. That is, wafers 20 and 21, which are satisfiedwith specifications of a device, are prepared. In the process 2, atleast one wafer, i.e., the wafer 20 is thermally oxidized to form anoxide film 30 having a thickness of about 0.1-2.0 μm on its surface. Inthe process 3, hydrogen ions or rare gas ions are implanted (hydrogenions are implanted here) into one side surface of the other wafer 21 toform a microcavity layer (implanted layer) 40 at an average penetrationdepth of the ions in parallel with its surface. And its implantationtemperature is preferably 25-450° C. The process 4 is the process suchthat a hydrogen ion implanted surface of the hydrogen ion implantedwafer 21 is brought into contact with a surface of the oxide film 30formed on the wafer 20 to be joined together. By contact of each surfaceof two wafers under a clean atmosphere at room temperature, both wafersare joined without an adhesive agent or the like.

Next, the process 5 is a delamination heat treatment process such thatthe joined wafer is separated into an upper silicon 28 (delaminationwafer) and a lower SOI wafer 10 (a SOI layer 25+a buried oxide film 26+abase wafer 27) by an implanted layer 40 which is a boundary. When heattreatment is performed at a temperature of 400-600° C. or more under aninert gas atmosphere, the joined wafer is separated into the uppersilicon and the lower SOI wafer by growth of hydrogen induced defectsgenerated during the delamination heat treatment process and strength ofrapid volume expansion due to vaporization of implanted hydrogen. Thedelaminated upper silicon 28 is removed.

Additionally, as one of ion implantation delamination method, it hasbeen developed recently that implantation ions are implanted in a stateof plasma so that the delamination process is performed at roomtemperature. In this case, the above delamination heat treatment isunnecessary.

Then, in the process 6, since the bonding strength between the wafersjoined with each other in the process 4 which is the joining process istoo weak to be used, as it is, in a device process, it is necessary tosubject the lower SOI wafer 10 to heat treatment to have a sufficientbonding strength. This heat treatment is preferably performed for thirtyminutes to two hours at a temperature of 1050-1200° C. under an inertgas atmosphere.

Additionally, it is possible to continuously perform delamination heattreatment in the process 5 and bonding heat treatment in the process 6,i.e., to continuously perform delamination heat treatment in the process5 and bonding heat treatment in the process 6 without removing the uppersilicon wafer to be delaminated in the process 5 from the lower SOIwafer, or it is also possible to perform heat treatment as combining theheat treatment in processes 5 with the heat treatment in processes 6.

Then, in the process 7, which is a touch polishing process,mirror-polishing is performed for a cleaved surface 50 so that the stockremoval of the polishing can be 70-130 nm, preferably about 100 nm.

After the above processes, a high quality SOI wafer 10 having a SOIlayer 25 with high uniformity of film thickness which is a layer havingno crystal defects can be produced.

Next, as to the method for producing a SOI wafer according to the methodB, in the process 1, two mirror-polished silicon wafers 22 and 23, whichare satisfied with the specification of a device, are prepared. In theprocess 2, at least one out of two wafers, i.e., the wafer 23 isthermally oxidized to form an oxide film 31 having a thickness of about0.1-2.0 μm on its surface. In the process 3, hydrogen ions or rare gasions are implanted (hydrogen ions are implanted here) from a surface ofthe oxide film 31 on the wafer 23 to form a microcavity layer (implantedlayer) 41 at an average penetration depth of ions in parallel with itssurface. And its implantation temperature is preferably 25-450° C. Theprocess 4 is the process such that a surface of the oxide film 31serving as a hydrogen ion implanted surface of the hydrogen ionimplanted wafer 23 is brought into contact with the silicon wafer 22,and by contact of each surface of two wafers under a clean atmosphere ata room temperature, both wafers are joined without a adhesive agent orthe like. Next, after the processes 5 to 7 conducted by the sameprocesses as the method A, a SOI wafer with a uniform film thicknesswhich have no crystal defects can be obtained.

In the present invention, the ion implantation dosage at the time whenhydrogen ions are implanted into one wafer in the process 3 of the ionimplantation delamination method is 5×10¹⁶ ions/cm² or more and lessthan 7.5×10¹⁶/cm². According to this, a width of a SOI island region ina edge portion of the SOI wafer, which is easily generated indelamination, can be reduced to 1 mm or less, and a SOI wafer of whichdefect density of LPD existing in a surface of the SOI wafer isextremely reduced can be produced. Further, when the implantation dosagefalls within the above range, the wafers can be stably delaminated.

Hereinafter, the present invention will be explained specifically inreference to the example, but the present invention is not limitedthereto.

EXAMPLE

By using a MCZ method (Magnetic field applied Czochralski method) inwhich crystal was pulled while applying a magnetic field, a siliconsingle crystal of which so-called grown-in defects were reduced waspulled while controlling pulling conditions. The ingot of the crystalwas processed in a normal manner, and mirror-polished silicon wafers (adiameter of 200 mm, crystal orientation of <100>, conductive type ofp-type, and resistivity of 10 Ω·cm) of which COPs were reduced in thewhole crystal were produced.

The surface of the wafers was measured in terms of COP by a surfaceinspection apparatus (SP-1, product of KLA-Tencor Corporation), and itwas found that no COP having a diameter of 0.19 μm or more existed. TheCOP (Crystal originated Particle) was a void type defect having a sizeof about 0.1-0.2 μm, which is one of grown-in defects.

Next, based on process conditions below, three SOI wafers per eachcondition were produced from the wafers produced above by an ionimplantation delamination method.

1) Thickness of a buried oxide film: 145 nm, and Thickness of a SOIfilm: 160 nm.

2) Conditions of hydrogen ion implantation; Implantation energy: 56 keV,Dosages: six standards of 4.5×10¹⁶, 5.5×10¹⁶, 6.5×10¹⁶, 7.0×10¹⁶,7.5×10¹⁶, and 8.5×10¹⁶ ions/cm².

3) Delamination heat treatment; Temperature: 500° C., Time: 30 minutes,and Atmosphere: an inert gas (Ar).

4) Bonding heat treatment; Temperature: 1100° C., Time: 2 hours.

5) Touch polishing; Stock removal: 100 nm.

After the SOI wafers were produced based on each dosage described above,the SOI wafers was measured about each SOI island region width andterrace width by a optical microscope. The results of the measurementsare shown in Table 1 and FIG. 2.

From these results, it was found that the width of the region where theSOI islands 42 exist was sharply expanded when the hydrogen implantationdosage was 7.5×10¹⁶ ions/cm² or more. Further, when the width of a SOIisland region was expanded, the width of the terrace 43 was alsoexpanded by the same tendency.

Further, it was found that when the hydrogen dozage was less than 5×10¹⁶ions/cm²′ formation of defects and strength of expansion due tovaporization of hydrogen is not sufficient since the hydrogenimplantation dosage was too small, a SOI layer was only partiallytransferred to a base wafer so that delamination on whole surface of thewafer fell into difficulties, and therefore a SOI wafer could not beproduced.

According to the above results, in order to narrow the SOI terrace widthand reduce generation of SOI islands in the hydrogen ion implantationdelamination method, the hydrogen implantation dosage is preferably lessthan 7.5×10¹⁶ ions/cm². Further, when the implantation dosage is lessthan 7.5×10¹⁶ ions/cm² and 5×10¹⁶ ions/cm² or more, the terrace widthand generation of SOI islands are almost the same level.

TABLE 1 Item Hydrogen ion SOI island Exam. implantation dosage regionwidth Terrace width No. [×10¹⁶ ions/cm²] [mm] [mm] 1 4.5 SOI layer couldnot be transferred to a base wafer 2 5.5 0.2 2.2 3 6.5 0.3 2.2 4 7.0 0.32.2 5 7.5 1.0 2.6 6 8.5 1.5 3.2

On the other hand, LPD densities in each surface of SOI wafers producedunder the conditions of each dosage were measured by an optical surfaceinspection apparatus (SP-1, product of KLA-Tencor Corporation). Themeasured size of LPDs was 0.19 μm or more, and the region in the edgeportion which is exclusive was 5 mm. The average of LPD densities in thewafer surfaces in three standards of the dosages are shown in FIG. 3. Itshows that when the implantation dosage is 7.5×10¹⁶ ions/cm² or more,LPDs are sharply increased, and when the implantation dosage is lessthan 7.5×10¹⁶ ions/cm², LPDs are suppressed at the same level.

Therefore, in the method of producing a SOI wafer by an ion implantationdelamination method, the hydrogen dosage to be implanted is preferably5×10¹⁶ ions/cm² or more and less than 7.5×10¹⁶ ions/cm², morepreferably, 5.5×10¹⁶ ions/cm² or more and less than 7×10¹⁶ ions/cm².

The present invention is not limited to the embodiment described above.The above-described embodiment is a mere example, and those havingsubstantially the same structure as that described in the appendedclaims and providing the similar functions and advantages are includedin the scope of the present invention.

For example, in the above-described embodiment, it is explained byexemplifying the case where a silicon single crystal wafer having adiameter of 8 inches is used. However, the present invention is notlimited thereto and can be applied to the case where a silicon singlecrystal wafer having a diameter of 4-16 inches or more is used.

1. A SOI wafer produced by an ion implantation delamination method inwhich two silicon wafers are bonded, wherein the SOI wafer is producedby the ion implantation delamination method and a silicon wafer isdelaminated by a delamination heat treatment, a width of a SOI islandregion in a terrace portion generated in an edge portion of the SOIwafer where a surface of a base wafer is exposed is narrower than 1 mm,and a density of pit-shaped defects having a size of 0.19 μm or moreexisting in a surface of a SOI layer detected by a LPD inspection is 1count/cm² or less.